Confirmed. Issue is exactly as bumpa describes. When I choose to install Braid, the SteamApps/common/Braid folder is created, but no files are downloaded. However, the "Verify Integrity of Game Cache" feature says "All files were successfullly validated."

These appear to be cases of the developers flagging the games as supported for Linux, but not actually updating them / uploading the necessary files. Steam sees them as supported for Linux, so it displays them in the library, but when it tries to grab the data, there's nothing there, so it just assumes you have everything and leaves it at that.

So just wait for the developers to provide the Linux versions I guess. I have noticed though that I can run Osmos from Steam just fine, and it's not officially in the supported games list.